Many well-known useful products contain set gypsum (calcium sulfate dihydrate) as a significant, and often as the major, component. For example, set gypsum is the major component of paper-faced gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings (see, e.g., U.S. Pat. Nos. 4,009,062 and 2,985,219). It is also the major component of gypsum/cellulose fiber composite boards and products, as described in U.S. Pat. No. 5,320,677. Products that fill and smooth the joints between edges of gypsum boards often contain major amounts of gypsum (see, e.g., U.S. Pat. No. 3,297,601). Acoustical tiles useful in suspended ceilings can contain significant percentages of set gypsum, as described, for example, in U.S. Pat. Nos. 5,395,438 and 3,246,063. Traditional plasters in general, e.g., for use to create plaster-surfaced internal building walls, usually depend mainly on the formation of set gypsum. Many specialty materials, such as a material useful for modeling and mold-making that can be precisely machined as described in U.S. Pat. No. 5,534,059, contain major amounts of gypsum.
Most such gypsum-containing products are prepared by forming a mixture of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water (and other components, as appropriate), casting the mixture into a desired shaped mold or onto a surface, and allowing the mixture to harden to form set (i.e., rehydrated) gypsum by reaction of the calcined gypsum with the water to form a matrix of crystalline hydrated gypsum (calcium sulfate dihydrate). This is often followed by mild heating to drive off the remaining free (unreacted) water to yield a dry product. It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum crystals, thus imparting strength to the gypsum structure in the gypsum-containing product.
All of the gypsum-containing products described above could benefit if the strength of their component set gypsum crystal structures were increased in order to make them more resistant to the stresses they may encounter during use.
To make wallboard panels a slurry, including calcium sulfate hemihydrate and water, is used to form the core, and is continuously deposited on a paper cover sheet moving beneath a mixer. A second paper cover sheet is applied thereover and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with a sufficient amount of the water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. The continuous strip thus formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture. Since each of these steps takes only minutes, small changes in any of the process steps can lead to gross inefficiencies in the manufacturing process.
The amount of water added to form the slurry is in excess of that needed to complete the hydration reaction. Some of the water that is added to the gypsum slurry is used to hydrate the calcined gypsum, also known as calcium sulfate hemihydrate, to form an interlocking matrix of calcium sulfate dihydrate crystals. Excess water gives the slurry sufficient fluidity to flow out of the mixer and onto the facing material to be shaped to an appropriate width and thickness. While the product is wet, it is very heavy to move and relatively fragile. The excess water is removed from the board by evaporation. If the excess water were allowed to evaporate at room temperature, it would take a great deal of space to stack and store wallboard while it was allowed to air dry or to have a conveyor long enough to provide adequate drying time. Until the board is set and relatively dry, it is somewhat fragile, so it must be protected from being crushed or damaged.
To dry the boards in a relatively short period of time, the wallboard product is usually dried by evaporating the extra water at elevated temperatures, for example, in an oven or kiln. It is relatively expensive to build and operate the kiln at elevated temperatures, particularly when the cost of fossil fuels rises. A reduction in production costs could be realized by reducing the amount of excess water present in set gypsum boards that is later removed by evaporation.
Another reason to decrease water is that the strength of gypsum products is inversely proportional to the amount of water used in its manufacture, especially in full density slurries. As the excess water evaporates, it leaves voids in the matrix once occupied by the water. Where large amounts of water were used to fluidize the gypsum slurry, more and larger voids remain in the product when it is completely dry. These voids decrease the product density and strength in the finished product.
Dispersants are known for use with gypsum to help fluidize the mixture of water and calcium sulfate hemihydrate so less water is needed to make flowable slurry.
Currently there are two main commercial options for gypsum slurry dispersants. 1) Polynaphthalene sulfonate dispersants and 2) Branched polycarboxylates.
Polynaphthalene sulfonate dispersants are well known and relatively cheaper, but have limited efficacy. Polynaphthalene sulfonate has good compatibility with starch, foaming agents, and clays. A production process for polynaphthalene sulfonates includes the following reaction steps: sulfonation of naphthalene with sulfuric acid producing b-naphthalene-sulfonic acid, condensation of b-naphthalene sulfonic acid with formaldehyde producing polymethylene naphthalene sulfonic acid, and neutralization of polymethylene naphthalene sulfonic acid with sodium hydroxide or another hydroxide. Depending on reaction conditions products with different characteristics are obtained.
U.S. Pat. No. 4,460,720 to Gaidis et al discloses a superplasticizer cement admixture for Portland based compositions formed from a low molecular weight alkali metal polyacrylate in combination with an alkali metal or alkaline earth metal polynaphthalene sulfonate-formaldehyde or an alkali metal lignosulfonate or an alkaline earth metal lignosulfonate or mixtures thereof.
U.S. Pat. No. 5,718,759 discloses the addition of silicates to mixtures of beta-calcined gypsum and cement. In the examples, lignosulfates or polynaphthalene sulfonates are used as water-reducing agents. The addition of pozzolanic materials, including silicates, is credited with-reducing expansion due to the formation of ettringite. The composition is suggested for use in building materials, such as backer boards, floor underlayments, road patching materials, fire-stopping materials and fiberboard.
Branched polycarboxylates are more expensive and more effective dispersants. However, they are less effective for stuccos with clay. Also, the branched polycarboxylates can reduce board strength and are surface active and can interfere with the foaming agent. Also, due to the side chains of the branched polycarboxylates, they cannot be mixed with the polynaphthalene sulfonate. U.S. Pat. No. 7,767,019 to Liu et al discloses embodiments of branched polycarboxylates.
Thus, there is a continuing need for new and improved set gypsum-containing products, and compositions and methods for producing them, that solve, avoid, or minimize the problems noted above. There is a need in the art to reduce the dosage of dispersants used in gypsum slurry while maintaining flowability of the slurry. Reduction in dispersant use would result in saving of costs spent on the dispersant